In LTE (Long Term Evolution) which is a next generation mobile communication system being a standard in 3GPP (Third Generation Partnership Project), a mobile station (UE: User Equipment) operates in an active state or in an idle state. In accordance with handover criteria (HO Criteria), the mobile station in the active state (also referred to as “connected state”) selects a destination cell, and moves thereto. On the other hand, the mobile station in the idle state (also called “idle mode”, “standby state/mode”, “power saving state/mode”) selects a destination cell in accordance with cell reselection criteria (CRS Criteria), and moves thereto. This process for movement is referred to as “cell reselection”.
Basically, a mobile station measures received power of radio wave or receiving quality thereof from a serving cell (it is also referred to as a cell in which a mobile station is located), and a neighbor cell (or neighboring cell, it is also referred to as “an adjacent cell”), and moves to a target cell from a serving cell. A cell of the movement destination (target cell) of a mobile station is determined based on the handover criteria and the cell reselection criteria using the measurement result of received power or receiving quality of a mobile station. In the handover criteria and the cell reselection criteria, types of the received power/receiving quality to be measured for determination, and structure of formula, and types of parameters which constitute the formula are different, respectively. When nearly satisfying a following formula (1), a mobile station moves to a target cell.PTarget cell>Pserving cell+HO_Margin  (1)
Note that, in the formula (1), PTarget cell is an index value of received power or receiving quality based on a reference signal from a target cell, and PServing cell is an index value of received power or receiving quality based on a reference signal from a serving cell. HO_Margin is a predetermined margin. Note that the types of parameters which constitute HO_Margin are different in the handover criteria and the cell reselection criteria. Moreover, with respect to a degree that the parameters which constitute each criterion affect to the range of the cell formed by the handover criteria or the cell reselection criteria, the degree changes with radio environment to which a system is actually applied. Therefore, in a radio environment to which a system is actually applied when starting an operation of a cell in accordance with the handover criteria and the cell reselection criteria, coverage defined by the handover criteria and coverage defined by the cell reselection criteria may not be matched. In the following illustration, a state in which the coverage defined by the handover criteria and the coverage defined by the cell reselection criteria are not matched, is expressed by a state in which the coverage is “not aligned”. Moreover, a cell which is not aligned is referred to as “non-aligned cell”.
FIG. 1 is a diagram in which the base stations A, B and C, which are adjacent to each other, form cells A, B and C, respectively, and a diagram depicting a case where the base station A is a non-aligned cell among the base stations A, B and C. In FIG. 1, (a) depicts coverage according to the handover criteria (HO criteria), and (b) depicts coverage according to the cell reselection criteria (CRS criteria), respectively. Since the base station A is a non-aligned cell as depicted in FIG. 1, the sizes of coverage according to the handover criteria and the coverage according to the cell reselection criteria are different.
In FIG. 1, it is assumed that a mobile station (UE (P1)) in the idle state under a control of a cell A is moving toward a cell B from a cell A, as depicted in FIG. 1. In this situation, the mobile station changes into the active state from the idle state at a boundary of the cell A and the cell B, and connects with a cell (cell A) which has received a radio wave in accordance with the cell reselection criteria (UE (P2)). Then, a case can be considered where a handover is performed to another cell (cell B) in accordance with the handover criteria right after establishing a connection with the cell A (UE (P3)). In other words, supposing that the base station A is aligned, if a mobile station changes into the active state and connects with the cell A afterwards, a mobile station would have continued the connection with the cell A unless the position is changed. However, since the base station A is not aligned, above-mentioned useless handover may occur.
FIG. 2 is a diagram in which the base stations A, B and C, which are adjacent to each other, form cells A, B and C, respectively, and a diagram depicting a case where the base station B is a non-aligned cell among the base stations A, B and C. In FIG. 2, (a) depicts coverage according to the handover criteria (HO criteria), and (b) depicts coverage according to the cell reselection criteria (CRS criteria), respectively. Since the base station B is a non-aligned cell as depicted in FIG. 2, the sizes of coverage according to the handover criteria and the coverage according to the cell reselection criteria are different.
In FIG. 2, it is assumed that a mobile station (UE (P1)) in the idle state under a control of a cell B is moving toward a cell A from a cell B. In this situation, the mobile station changes into the active state from the idle state (UE (P3)) right after it moved to an area under a control of the cell A according to cell reselection criteria (UE (P2)), and establishes a connection with a cell (cell A). Then, a case can be considered where a handover is performed to another cell (cell B) in accordance with the handover criteria soon afterwards (UE (P4)). In other words, supposing that the base station B is aligned, if a mobile station changes into the active state and connects with the cell A afterwards, a mobile station would have continued the connection with the cell A unless the position is changed. However, since the base station B is not aligned, above-mentioned useless handover may occur.
FIG. 3 is a diagram in which the base stations A, B and C, which are adjacent to each other, form cells A, B and C, respectively, and a diagram depicting a case where non-aligned cell does not exist. In FIG. 3, (a) depicts coverage according to the handover criteria (HO criteria), and (b) depicts coverage according to the cell reselection criteria (CRS criteria), respectively. In FIG. 3, at each cell, the coverage defined by the handover criteria and the coverage defined by the cell reselection criteria are matched. In this case, a useless handover does not occur right after a mobile station changes from the idle state to the active state.
Currently, 3GPP is standardizing not only constituting parameters of above-mentioned handover criteria and cell reselection criteria but also SON (Self-Optimization Network) which addresses automated optimization of the various parameters. In MRO (Mobility Robustness Optimization) which is one of use cases of the SON, a problem is disclosed in which each criterion is to be adjusted to be matched by optimizing each parameter which constitutes the handover criteria and the cell reselection criteria, and occurrence of a useless handover is to be reduced.
Related art is disclosed in 3GPP TS 36.902, “Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Self-configuring and self-optimizing network (SON) use cases and solutions”.